1. Field of the Invention
The present invention relates to a digital transmission apparatus and, more particularly, relates to construction of a VLAN for carrying out communications between groups in an asynchronous network using digital transmission apparatuses.
2. Description of the Related Art
In an asynchronous network such as the Ethernet and an enterprise network using SDH (Synchronous Digital Hierarchy) apparatuses and the like, a VLAN is constructed. FIG. 16 is a diagram showing typical construction of a VLAN in an Ethernet. As shown in FIG. 16, in an asynchronous network such as the Ethernet, terminals 2#Ai where i=1, 2 and so on form group A whereas terminals 2#Bi where i=1, 2 and so on form group B. Groups A and B generally implement the VLAN as follows. For each network, in a bridge serving as a data-outputting source or a router 4#i where i=1, 2 and so on, a group identification such as a VLAN-Tag is assigned to an IP or MAC frame. In a bridge or a router 4#i where i=1, 2 and so on provided on the data reception side, on the other hand, an IP or MAC frame other than its own group is discarded to make exchanges of data with groups other than its own group impossible.
FIG. 17 is a diagram showing typical construction of a VLAN in a synchronous network using SDH transmission apparatuses. In a synchronous network using SDH transmission apparatuses and the like wherein terminals 2#Ai where i=1, 2 and so on form group A whereas terminals 2#Bi where i=1, 2 and so on form group B, and a VLAN is implemented by using groups A and B, communication path A in group A and communication path B in group B set the same band. The SDH transmission apparatuses 4#i where i=1, 2 and so on each carry out cross connection in a band assigned to the path in accordance with path setting to enable only communications within the same group. However, the conventional construction of a VLAN has the following problems:
1: In an attempt made to accommodate a plurality of asynchronous networks such as the Ethernet in groups including SDH transmission apparatuses, it is necessary to set a path for each group. It is thus necessary to assign maximum bands to as many pathes as the groups so that wasteful bands are acquired inadvertently.
2: When a plurality of asynchronous networks such as the Ethernet are accommodated in an SDH transmission apparatus using a path, it is necessary to separately provide a routing apparatus such as a bridge or a router for adding and deleting a tag to and from data on the path between the SDH transmission apparatus and an asynchronous network for each group. Thus, implementation of the network requires a high additional cost.
3: In an attempt to implement a communication between different groups with a routing apparatus such as a router or a bridge setting a tag for each group, it is necessary to add a plurality of group tags so that management of groups becomes difficult.
4: An attempt to carry out a communication between groups in an SDH transmission apparatus entails a mechanism for carrying out operations including extraction of only necessary data from received data and transmission of the extracted data to another path after incoming data is received. Thus, the cost of the apparatus increases.